AC commercial power is often used as a primary power source to power communication and data processing equipment which utilize stored program controls and solid-state integrated circuit technology. These circuits are generally very sensitive to any variations of the input power signal from its desired standard waveform. Commercial AC power waveforms are subject to many variations from the standard waveform due to the demands of other users on the power line and other extraneous factors.
Undesirable power signal variations causing problems include overvoltage and undervoltage conditions, signal outages, and signal transients such as voltage spikes. These power signal variations may alter the stored data or the switching signals and, in extreme cases, may damage the solid-state circuitry. Transient and momentary outages may cause undetected damage in data areas which are not immediately apparent and eventually cause costly shutdowns due to damaged circuitry, disrupted communications, or introduce errors in computations.
To avoid these aforementioned problems, uninterruptible power supplies are utilized to isolate variations in the AC power signal from the equipment being powered and to supply continuous power to an output load regardless of the actual performance of the basic input commercial AC power signal.
One uninterruptible power supply system providing signal variation and signal outage protection to solid-state equipment is disclosed by H. Fickenscher et al in U.S. Pat. No. 4,010,381, issued Mar. 1, 1977, and assigned to the same assignee as the instant application.
This uninterruptible power supply couples two power sources through a single highly reactive transformer structure to supply uninterruptible power to a load to be energized. A primary power source, typically supplied by a commercial utility, customarily continuously supplies power to the output. A secondary or reserve power source is activated to supply power to the load only upon failure or degradation of the primary power source.
Power flow through a highly reactive transformer is a function of the phase angle between an input signal and the output signal. To achieve high efficiency, the reserve power source is continuously operated in an idling condition, that is, the inverter switches are timed to operate so that the output signal of the inverter is substantially in phase with the output signal of the power supply. Hence during idling, no power flows from the reserve source to the output load. Upon failure of the primary source, the reserve power source responds immediately to supply power to the output load.
Since uninterruptible power supplies are frequently used to supply power to data processing type circuits, the reserve power source must act quickly to supply power whenever the primary power source experiences either a permanent or temporary failure in order to maintain the integrity of information in the data processing circuit.
To operate efficiently, the controller for this power supply must accurately monitor and properly measure all input and output signals and respond promptly should an input or output signal become unacceptable.